It is essential to determine the moisture content in the articles especially relating to articles used in food industry. The knowledge about the moisture content is essential factor for selecting the storage conditions and the safe storage period of a particular sample. Moisture content of various industrial materials like rubbers, foam, soap cakes, pharmaceutical products is an important factor for storage, shelf life and quality control.
Measurement of dielectric properties is important in the industrial processes involving polymers, rubbers, ceramics and plastics for the quality control as well as correct chemical compositions of the product in the liquid, semisolid or solid state.
Standard gravimetric laboratory tests are tedious and require several hours for completion, also they are destructive in nature. Number of conductivity based and capacitive techniques measure average moisture in bulk materials and need particular size of the sample to be measured. Moisture meters with penetrating probes are suitable for semisolid or grainy material or liquids only. Waveguide resonant cavity techniques have been widely used to perform non-destructive moisture measurements as well as complex permittivities of semiconductor materials. The waveguide cavity technique is associated with various difficulties like loading and unloading of samples even though it is considered as an accurate technique.
Moisture measurements in materials like rubber lining need a sensor which can measure moisture with surface contact only. The present technique can also be used for the measurement of thickness of ice formed on the wings of the aircraft of a space shuttle.
The moisture content of grains, seeds and food products such as wheat, rice, coffee is an important factor for the storage of grain, determination of the time of harvesting, marketing and processing (S. O. Nelson, V. K. Chari Kandala and Kurt C. Lowrence. "Moisture determination in single grain kernels and Nuts by RF Impedance Measurements". IEEE Trans. on Instrumentation and Measurements, Vol 41, No. 6, Dec. 1992 and Andrezej W. Kraszevaski, "Microwave Aquametry--Needs and Perspective," IEEE Trans. on Microwave theory and techniques, Vol 39, No. 5, May 1991)
Moisture measurement is performed on the samples from yields of a cereal or pulse product to correct the yield figure. The error of moisture content of 1% in one ton of yields can cause an error of 10 Kg. Forecasting of the yields is important for enabling to take an action against the unbalanced demand and supply of the cereal products. It is extremely important to store the grain stock only after the moisture contents have been measured to ensure the prevention of insect manifestation. Measurement of moisture of materials is important in several industrial processes such as production of soaps. Similarly moisture measurement for liquids such as fruit products is essential. Measurement of moisture of materials is important in several industrial processes such as production of soaps, powders, biscuits, sugar syrups and for oil based products such as margarine.
The prior art cites references to different methods of quantifying the moisture contents. Moisture meters/sensors have been used which include the standard gravimetric laboratory tests which are tedious and require several hours and days for completion. Conventional electric and electronic moisture meters measure average moisture in bulk grain sample. The average does not provide any information on the range of moisture in the individual grains or seed moisture content [A. W. Kraszevaski and S. O. Nelson, "Moisture content determination in Single Peanut Kernels With a Microwave resonator," Peanut Science, (1993) 20:27.31.]
Microwave moisture sensor (U.S. Pat. No. 4,991,915, issued on Feb. 12, 1991) works on the principle of absorption of the microwave energy into the material. This type of sensor is useful in case of high moisture content as the attenuation characteristics of microstrip are sensitive to the heavy losses in the material under test. U.S. Pat. No. 5,313,167 issued in 1994 teaches coaxial line for introducing microwave energy with the detector coaxial line kept at a distance.
The waveguide cavity measurement technique is associated with the difficulties of loading and unloading of individual kernels and placing them accurately inside the cavity. The number of direct current conductivity and capacitance measurement based moisture sensors available in the market can handle bulk grain samples only where it is impossible to measure the range of moisture contents or dielectric parameters of the grains or materials. They are also affected by the air gaps in the grain samples and conductivities of the grain covers as well as non-uniform shapes. The conductivity based moisture meters need constant pressure to be applied to the bulk under measurements for the validity of look up tables. Hence they are subject to the human errors.
Low frequency techniques are associated with the contributions due to the ionic conductivities of the moist grain and hence are subjected to inherent errors. These type of sensors are based on the power measurement and are totally dependent on the calibration of the materials on the custom made sensor and totally dependent on the dynamic performance of the detector system and its readout system performance.
U.S. Pat. No. 4,408,128 works on the principle that the electric capacity of a sample held between positive and negative electrodes is measured and the value of moisture content thereof is indicated on the basis of the correlation studies in advance between moisture content and electrical characteristic.
Due to the variation in the grain size, a single moisture meter fails to serve the purpose of all cereal products. It is therefore necessary to provide a moisture meter having correlation's calibrated for five respective cereal products according to the type of grain size. The apparent difference in the water content value caused by the difference in the temperature at the time of measurement makes correction by temperature desirable.
A method where a plurality of keys designating the type of sample with programmable EPROM read only memory capable of erasure and rewriting for storing parameters for computation are provided, is also known in the art. The EPROM is fed with the precalibrated values of capacitor voltage and moisture content. The indicator indicates the moisture content in the digital form. However, in this patent it is not clear whether the instrument measures moisture in a single grain or in a bulk of sample, e.g., 100 gm of grains. The description of the actual sensor geometry and dimensions is not drawn or given. The drawbacks associated with this patent specifically relate to the fact that the single type of grain like wheat or rice has variation in the grain size. The capacity is a function of thickness and area and permittivity of the material between the plates. The said instrument does not give effective variation in the capacity with the size and weight of individual grains. If they measure 100 g of grains, air gaps between the grains effect the reading thereby indicating a lower moisture content then the actual true value.
Another U.S. Pat. No. 5,646,537 describes a time domain reflectometry waveguide assembly for measuring the moisture content in a medium and comprises calculating the apparent dielectric constant value of said medium based on a time delay measured in response to said detectable characteristic reference reflection, and correlating the apparent dielectric constant value with data reflecting the moisture content of the medium.
Another U.S. Pat. No. 5,420,517 defines a time domain reflectometry waveguide assembly for measuring the moisture content in a medium.
Microwave moisture sensor (U.S. Pat. No. 4,991,915, issued on Feb. 12, 1991) works on the principle of absorption of the microwave energy into the material. This type of sensor is useful in case of high moisture content as the attenuation characteristics of microstrip are sensitive to the heavy losses in the material under test. These sensors are based on the power measurement and are totally dependent on the calibration of the materials on the custom made sensor and totally dependent on the dynamic performance.
U.S. Pat. No. 5,313,167 issued in 1994 teaches coaxial line for introducing microwave energy with the detector coaxial line kept at a distance.
The waveguide cavity measurement technique is associated with the difficulties of loading and unloading of individual kernels and placing them accurately inside the cavity. The number of direct current conductivity and capacitance measurement based moisture sensors available in the market can handle bulk grain samples only where it is impossible to measure the range of moisture contents or dielectric parameters of the grains or materials. They are also affected by the air gaps in the grain samples and conductivity's of the grain covers as well as non-uniform shapes. The conductivity based moisture meters need constant pressure to be applied to the bulk under measurements for the validity of look up tables. Hence they are subject to the human errors.
Low frequency techniques are associated with the contributions due to the ionic conductivity's of the moist grain and hence are subjected to inherent errors. These type of sensors are based on the power measurement and are totally dependent on the calibration of the materials on the custom made sensor and totally dependent on the dynamic performance of the detector system and its readout system performance.
The sensors which measure moisture in the 100 g of grain samples are incapable of measuring a range of moisture in the grain stock. Mixing of grains containing different moisture contents lead to the degradation of the complete stock. Hence it is important to measure moisture in individual kernels. The same applies to the moisture measurements before harvesting for the estimation of the national or regional yield to predict the national needs for import or export. Similarly moisture measurement of liquid samples such as sugar syrups, milk products, fruits such as grapes, apples, watermelon etc. require determination of precise moisture content.
Moisture measurement in industrial products like rubbers, foams, soaps is a commercially important technique for the quality control. Number of techniques in the market include conductivity, capacitance, frequency shift, time domain and nuclear techniques. Most of these techniques are destructive and demand machining of samples to peculiar geometry of insertion of probes into the material.
The drawbacks of the conventional methods for determination of moisture content hinder the exact determination of the moisture content. The need has therefore been realised for a method and an instrument for determining the moisture content in various samples which overcomes the problems mentioned above. Time domain technique is a powerful measurement tool for the dielectric constant of the sample under test.